Electro-optic displacement device

ABSTRACT

A device for determining and measuring the direction and extent of displacement of a regularly divided linear or circular scale comprises means for illuminating the scale over a plurality of divisions and detector means for imaging the illuminated portion of the scale upon an image divider which directs corresponding portions of the scale image to different ones of a plurality of photo-electric cells. The light thus incident upon the various photocells represents an average over a number of scale rulings and regularly varies with displacement in phase-displaced relationship among the photocells to provide a plurality of signals which can be employed in a directional electronic counting device.

United States Patent [191 Erickson Dec. 11, 1973 ELECTRO-OPTICDISPLACEMENT DEVICE Kent E. Erickson, Brookside, NJ.

[73] Assignee: Keuffel & Esser Company,

Morristown, NJ.

[22] Filed: Mar. 1,1972

[21] Appl. No.: 230,758

[75] Inventor:

Primary Examiner-Benjamin A. Borchelt Assistant Examiner-S. C. BuczinskiAtt0rneyJ. Russell Juten et al.

[57] ABSTRACT A device for determining and measuring the direction andextent of displacement of a regularly divided linear or circular scalecomprises means for illuminating the scale over a plurality of divisionsand detector means for imaging the illuminated portion of the scale uponan image divider which directs corresponding portions of the scale imageto different ones of a plurality of photo-electric cells. The light thusincident upon the various photocells represents an average over a numberof scale rulings and regularly varies with displacement inphase-displaced relationship among the photocells to provide a pluralityof signals which can be employed in a directional electronic countingdevice.

1 Claim, 8 Drawing Figures PATENTEI] DEC 1 l 1975 SHEETZUFZ DISPLACEMENTF/GZ4 J 2:1} DISPLACEMENT ELETRO-OPTIC DISPLACEMENT DEVICE BACKGROUNDphoto-electric means for generating electrical signals useful in anelectronic counter, such signals being representative of theinterruption of illuminating light by the scale graduations withmovement of the scale.

The simplest of such systems employs, for example, means of illuminationwhich projects a narrow light beam equivalent in lateral dimension tothe width of the graduations on the scale, in combination with aphoto-electric detector having a field of view similarly limited indimension. Movement of the scale thus provides a shuttering effect whichcan be regularly counted from the signal output of the photo-electricdetector. Such systems are, of course, practically limited by thedimension of graduation width and are not significantly useful where itbecomes desirable to ob tain the measurement of smaller graduationdisplacements.

improvement in the capability of measuring devices of this type toindicate smaller subdivisions of scale graduations has been obtained bythe use of pairs of scale divisions or grating patterns to generatemoire fringes or fringe patterns from which photo-electric cells cangenerate fluctuating electrical signals. Systems of this type are wellknown and have been described, for example, in US. Pat. No. 3,454,777.

Variations in such systems have further increased the sensitivity ofmeasurement, such as in the procedure of imaging a grating scale uponitself to provide the signal generating moire fringepattern at thephotocell detector. Refinements of this type improved the matchingregularity in the pattern-forming gratings, and additionally improvedthe sensitivity of systems by effecting a counter-current movementbetween the primary and secondary grating patterns. Systems of this typehave been described in U.S. Pat. Nos. 3,245,307 and These previoussystems suffered, however, from inherent limitations and requiredcompromises in use which severely restricted their versatility andeffectiveness. The simplestline by-line counting systems were forallpractical purposes unidirectional; that is, they were incapable ofdistinguishing the direction of movement of the scale, since theyutilized single detectors and thus merely counted the number of scaleunits displaced, regardless of the direction of displacement. Anadditional problem inherent in these simple systems derived from thefact that each of the individual scale graduations, or indicia, wasviewed in turn by the photo-electric detector and thus, unless perfectrepetition of indicia size and position was maintained, the

output signals varied uncontrollably. Also, since an exposed scale inpractical use is susceptible to abrasions and accumulated dirt; it wasnearly impossible to insure the required regularity in scale graduationsover a useful length of the scale.

The fringe-generating systems provided means for utilizing a pluralityofdetectors and thereby obtaining multiples of signal output'which couldbe employed in directional displacement indicators. Even in suchsystems, however, there remained the disadvantage that distinctlydifferent ones in the array of scale graduations were associated at agiven time with each of the plurality of detectors. These systems,therefore, remained vulnerable to isolated indicia variations or marringof the scale. In an attempt to avoid this problem by distributing thelight from a number of the scale indicia among the plurality ofdetectors, additional elements such as beamsplitters were introducedinto the systems with the further disadvantageous results thatefficiency of light utilization was reduced and the arrangements becamemore and more complex with the introduction of each new element. Notonly were the requirements of perfection in both the manufacture andalignment of components made more extreme, but also there were practicallimitations imposed upon the use of such systems, such as, for example,the limitation against use in conditions of vibration and shock. Sincethe moire fringe patterns may be displaced not only by movement in thedirection of desired measurement, but also in other vector directions,rigid control of the positioning and stability of arrangement had to bemaintained in order to provide a useful device.

SUMMARY The displacement measuring system and arrangement of the presentinvention eliminates many of the disadvantages of prior systems andprovides a compact, rugged and accurate means for generating a pluralityof signals which may be employed in determining and indicating bothdirection and extent of minute movements.

The invention, in particular, provides a system which utilizes thesimplicity and stability of design associated with the earliest-noteddevices, yet includes means for distributing proportionate parts of thelight of a plurality of scale periods among a number of photocells. Bythus accumulating at each photocell like portions of light from apluralityof scale periods, the invention ensures that the influence onthe light signal of any isolated error or mar in the scale becomesinconsequential, since such an error is submerged in the substantiallygreater accumulated light from the numerous other like parts of thescale.

The present invention allows the use of a scale which may besufficiently coarse in its graduations to be readily produced byprinting methods. Such a scale, depending upon its intended use, maytake the form of a steel tape or other dimensionally stable carrierhaving a regular pattern of transverse rulings along its length. Ifdesired, the carrier may be substantially transparent with the rulingsbeing in the form of opaque areas. In use, the scale is attached to awork piece or other surface and displacements in the longitudinaldirection of the scale, that is transverse to the rulings, aredetermined by the system.

An illumination module, including an incandescent lamp and a simple lenssystem, is arranged to project a beam of light upon the surface of thescale over a sufficient dimension to illuminate a plurality of thescalerulings. A second detection module comprises simple optics capable ofimaging the illuminated portion of the scale at an image plane and,situated substantially at that image plane, an image divider.

Although this latter element will be hereafter described in greaterdetail, it is sufficient here to note that the element comprises aplurality of sections which are disposed in substantial alignment withthe graduation pattern constituting the scale image, the sections beingrepeated transversely of the image graduations and having a fixeddimensional relationship with respect to the period of the image.Depending upon the selected lateral dimension of the sections, thedividing means separates each period of the image into a fixed pluralityof segments. The proportion of light comprising each of the segments inany one image period varies with the relative positions of the scale andthe dividing means; however, with respect to corresponding segments ofthe numerous periods of the image, each comprises an equal amount oflight. In addition to effecting this scale image division, the elementserves a further role of directing the light comprising correspondingsegments of the scale image to respective ones of a plurality ofphoto-electric cells. In this manner there is accumulated at eachrespective photocell the same proportionate parts of the lightconstituting each segment of the recurring periods of the scale image.As noted, such accumulation renders innocuous any scale error residingin a single period segment.

As displacement occurs in a direction transverse to the scalegraduations, that is, in the direction of periodicity of the scale, theimage of the sclae at the imagedividing means moves across the face ofthat element in a direction transverse to the noted sections anddifferent proportionate parts of the scale image are directed to therespective photocells with the result that the corresponding intensityof electrical signal output from each of the photocells varies in aquasi-sinusoidal pattern with regular displacement of the scale. Due tothe fact that the sections of the image-dividing means recurr in aregular manner across the image plane, the respective patterns of outputsignal intensity from the various photocells, while retaining a generalsimilarity in periodicity, are displaced in phase.

The electrical signals generated by the photocells in the present devicemay be readily employed in known electronic directional counting devicesand a display may readily be provided which indicates the extent ofmovement of the scale image, hence the displacement of the scale. Suchdisplays may be employed directly in distance measuring or may beincorporated further as control means for automatic tooling devices andthe like.

DRAWINGS FIG. 1 is a schematic representation of the detection module adevice according to the present invention, viewed in a planeperpendicular to the scale rulings; as at I-l of FIG. 2;

FIG. 2 is a schematic view of the detection module in a plane parallelto the scale rulings, as at 22 of FIG. 1;

FIGS. 3 is a schematic representation of the division and lightdistribution of a portion of a scale image at a faceted reflectivesurface of an image divider;

FIG. 4 is a schematic representation of the relative dispositions ofportions of a scale image and faceted image divider surface during aperiodic displacement cycle of the scale image;

FIG. 5 is a representation of the patterns of electrical signal outputfrom a plurality of photocells in an embodiment of the presentinvention;

FIG. 6 is a representation of a preferred embodiment of the presentinvention as viewed in a plane parallel to scale rulings;

FIG. 7 is a view of the preferred embodiment of FIG. 6 taken along 7-7;and

FIG. 8 is a schematic representation of a portion of a preferred imagedivider of the present invention.

DESCRIPTION Scales employed in the present device may be readilyprepared by printing or otherwise marking upon a desired carrier surfacerulings or graduations of optically discernible material.Light-absorbing compositions or metallic deposits serve well in thisregard. The rulings or scale indicia are disposed in a substantiallyparallel array along the length of a tape or, when it is desired tomeasure angular displacement, along the periphery of a circular disc. Asshown in FIG. 1, each of the indicia 11 is of a pre-selected width andis separated from its neighboring indicia by a space of substantiallyequal width, thereby establishing a periodicity, d, along the extent ofthe array of indicia. Depending upon the ultimate use of the device,indicia 11 may be placed upon a carrier which is opaque, in which eventillumination of the scale is effected at the same surface observed bythe detection system. On the other hand, it may be desirable in someembodiments to illuminate the scale indicia from the side opposite thatat which the detector is located, in which event the indicia may becarried upon a transparent base such as glass, plastic or the like.

A detection module of the present device, as shown in FIGS. 1 and 2,generally comprises optics 13 situated along the optical axis 12 of themodule and of such dimensions as will image scale indicia 11 at an imageplane, normally at nearly unit magnification. This degree ofmagnification is by no means critical and may be selected by theultimate system designer without major restrictions. However, the unitmagnification is sufficient to provide a mechanically simple andfunctionally reliable system.

Substantially at the image plane there is positioned an image divider 15which serves to separate the image of scale indicia 11 into a series ofsegments running generally parallel to the direction of rulings, orindicia, 11. By virtue of the construction of the image divider, as willlater be discussed in greater detail, the light of recurring ones of thevarious segments of the scale image are directed to different ones of abank of photocells 17; for example, three such photocells P P and P asshown in FIG. 1. Image divider 15 may additionally be employed toprovide such further shift in the disposition of the optical axis of themodule (as in FIG. 2) as may be desired in the selected mechanicaldesign of the system.

The present invention may be applied to the use of any desired pluralityof photocells; however, it has been found particularly useful to employthree such photocells. In this manner an effective balance ofsensitivity and physical size isv achieved, and the system is welladapted to the use of a reversible counter such as described in US. Pat.No. 3,271,676. The regular threefold division of the period of the scaleimage effects photocell signal outputs which are phase-varied, one fromthe other, by and thus provides a useful combination ofelectronic'signals which may be readily employed to achieve common modeelimination and obtain signals with more truly AC characteristics.

In the system schematically represented in FIGS. 1 and 2, image dividercomprises a generally spherical reflective surface which is ofappropriate dimensions and disposition, as seen in FIG. 2, to focus thelight comprising the scale image at photocell bank 17. The role ofimagedivider may be accomplished by reflector 15 through the formationof a plurality of facets in the reflective surface. As shown in FIG. 1,the facet surfaces f,, f f are longitudinally disposed with reference toindicia. .11, thatis, lie generally parallel to the, length of indicia11. Each facet surface is additionally individually situated at an anglewith respect to its neighboring facets in such a manner that each set ofthree neighboring facets focuses scale image light in the direction ofdifferent ones of the plurality of the three photocells in bank 17. Thisarrangement of facet surface sets is continued across the face ofdivider 15 in any number of desired set repetitions.

It should be noted in general that the relative dimensions employed inthe schematic representation of FIGS. 1 and 2 are by no means to scaleand have been for the most part grossly exaggerated for purposes ofclarity of discussion. Further, it will be understood that while thepresent description refers primarily to the use of" reflecting surfacesas the facet elements in the image-dividing means, refractive elementsmay be similarly utilized. In such a latter construction the photocellbank would, of course, be situated on axis beyond the dividing means.Apparent also should be the fact that combinations of refractive andreflective element groups can be practically employed, if desired. Thus,for example, one or more of the respective elements in each facet setmay be refractive with the remaining elements being reflective.

The effect of each of the facet sets may be seen more clearly byreference to FIG. 3. One set of facet surfaces f f f lie substantiallyat the image plane of scale image 31 which has a period, d, of apreselected size relationship to scale period, d. As shown in FIG. 1,the optical system includes means 14 for limiting the aperture of lens13 inorder to achieve sufficient depth of focus to accommodate theslight actual curvature of surface 15, that is, to insure that scaleimage 31 will be substantially in focus over the effective width ofsurface 15, as well as toallow practical tolerance in positioningelements of the device. Again for the sake of pictorial clarity, scaleimage 31 (FIG. 3) has been displaced from the surface of divider 15, yetit should be noted that the relative lateral disposition of image 31 andthe faceted surfaces for an arbitrarily preselected stationary positionof the scale has been retained.

With a preferred lateral facet set dimension substantially equal to theperiod, d, of the scale image, the light comprising the image of thespace between image indicia 31, 31 falls upon the whole of facet f uponone-half the width of facet f and not all all upon facet f As a result,facet f focuses a unit of incident light in the direction of photocell Pfacet f, focuses onehalf as much light'in the direction of photocell Pand facet f reflectsno light at all in the direction of photocell P,.The outcome of the instant lateral relative disposition betweensurfacel5 and the scale image is,

therefore, that photocell P P and P, are respectively illuminated, andconsequently provide output signals in the ratio of 2:120. It will thusbecome apparent that with a transverse shift in the location of thescale image at surface 15, as occasioned by a displacemtnt of the scaleand indicia 11, more or less light is incident upon the facet series f ff to be concurrently focused at photocells P P P The effect of thedisplacement of the scale can further be seen with reference to FIG. 4.With an initial relative lateral position, 0, of indicia image 31 withrespect to a facet set of a section 45 of the image divider similar tothat shown in FIG. 3, light is incident only upon facets f and f and inthe approximate ratio of 12. Considering displacemen of the sc a direc;.7,

tion to effect downward movement of scale image 31 (FIG. 3),displacement of the image occurs in the indicated direction in FIG. 4.Upon such displacement of the scale image through one-third of the scaleimage period, d'l3, indicia image 31' now becomes aligned with the facetset of section 45 such that light is incident only upon facet f and f;in a ratio of 2:1. Further displacement through two-thirds of a period,2dl3, of the scale image results in displacement of indicia image to aposition of 31" with light being incident only upon facets f and f inthe ratio of 1:2. Still further displacement through one complete imageperiod, d, concludes a displacement cycle with the respective facets fand f being lighted to the extent originally noted.

Since light incident upon the respective facets f f f of each of aplurality of sets on a surface 15 is directed to correspondingphotocells P P P with a resulting similar ratio of signal output fromthese photocells, a quasi-sinusoidal pattern of output signals from thephotocells would appear with respect to each image displacement cycle,d, as shown in FIG. 5. Utilizing each of the signals, phase-displaced by120, in an electronic counter described in U.S. Pat. No. 3,271,676provides a means for determining the net actual displacement of thescale.

From the foregoing it can be seen that by virtue of the describedconstruction of the image-divider, the invention accomplishes directlyin this simple unitized element the separation and distribution of thescale image light in such a manner that each of numerous scale periodscontributes to the total light signal incident upon each of the variousphotocells in the system, and thus eliminates the requirement forauxiliary beamsplitter elements which would otherwise necessitatecomplex assembly to ensure proper alignment and registration ofindividual scale-dividing grid elements.

PREFERRED EMBODIMENT An embodiment of the present invention, shown inFIGS. 6 and 7, has been designed primarily for one-side operation, thatis, to be used in conjunction with an opaque scale, such as a ruledsteel tape. In this form, the device is adapted to modular constructionfor simple and rugged assembly at manufacture and requires minimumadjustment, such as disposition with respect to the scale, at the timeof use in the field. Illumination module 60 and detector module aredesigned for economical mass productionand comprise bodies 64, 74 oftransparent thermoplastic material which may be molded individually forlater assembly, or may be manufactured as a single unit.

Illumination module 64 is of an optical quality acrylic plastic withaccommodation for securing a light source, such as incandescent lamp 63,on the optical axis of molded lens portion 66. Detector module 70 issimilarly molded from acrylic plastic with image-dividing surface 75situated on the optical axis of molded lens portion 76. Accommodation isprovided for mounting photo-electric cells 77 along the optical axis ofthe unit as displaced by dividing surface 75.

The scale employed in this embodiment comprises a steel tape 62 havingdisposed in regular array on one surface graduation indicia 61. Theseindicia are of a lateral width substantially equal to the space betweenindicia and establish a periodicity, d, of about 0.3mm in thelongitudinal direction of the tape. This dimension has been found to bewell within the tolerances which can be maintained in mass production.

Lens 66 of the illumination module 60 of the device is preferably ofastigmatic, or toric, dimensions. The curvature of lens 66, as shown inFIG. 6, is preferably such as to image light source 63 at the scaleindicia over a limited portion of the length of indicia 61. Thislimitation on the longitudinal dimension of the indicia image reducesthe susceptibility of the system to errors occasioned by twist of thedetection module about its optical axis. The second curvature of lens 66is not shown in the present drawings, however, it is preferably of suchgreater radius as to focus the light from source 63 at a distancesubstantially equal to that between lenses 66, 76. In the course of itspath in this second plane, the light beam illuminates a preselectedplurality of scale graduation periods, approximately 12 in the presentpreferred embodiment.

Lens section 76 of detection module 70 is spherical in nature and ofsuch dimensions as to image the illuminated portion of the scale indiciaat dividing surface 75. In order to insure sufficient depth of focus toprovide effective imaging over a significant portion of surface 75, forexample, over at least 12 facet sets in this embodiment, the aperture oflens 76 is reduced as at 76 (FIG. 7).

Silicon photodiodes are employed as the photoelectric detectors 77 andare situated at the plane in which lens 76 is imaged by generallyspherical reflective surface 75. In this manner the light directed uponthe aperture of lens 76 by illuminator module 60 is directed at optimumintensity upon the plurality of photocells 77. I

The modular construction of the preferred embodiment of the presentinvention permits inexpensive manufacture and assembly, and ensurespractical alignment of the lens and reflector elements of the device.The final adjustment in the assembly of the device, that is, thepositioning of the combined modules with respect to the scale may bereadily accomplished in the field, and, due to the noted relationshipsbetween the individual elements of the modules, the positioning andassembly may be accomplished without excessive attention to otherwisecritical placement of elements.

The general structure of a preferred reflective image divider of thepresent invention is shown in FIG. 8 where surface 85 comprises aplurality of sets of facets f f f which may, in essence, be individuallyconsidered as deriving from recurring sections taken from sphericalsurfaces 81, 82, and 83. Theselatter reference surfaces are ofsubstantially equal radius and each is disposed with respect to opticalaxis 80 in such a manner as would direct light rays from lens 76 (FIG.6) to each of the respective associated photocells P P P For example,each of spherical surfaces 81, 83 is positioned with its respectiveoptical axis shifted at an angle, a, from the optical axis 80 of surface82.

Actual dimensioning of surface 85 is accomplished by dividing thereference surfaces into the desired number of substantially equalsections and transferring selected sections in sequence to referencesurface 82 to form the composite faceted reflecting image-divider 85.For example, as shown in FIG. 8, segments f, of surface 81 aretransposed to surface 82 where they constitute facetsf Similarly,segments f;, of surface 83 form facets f of reflector 85. In practice,surface 85 forms the mold surface element for reflector 75 of detectormodule (FIG. 6). Such a surface formed at the time of molding module 70is subsequently silvered at the exposed outer surface to form theimage-dividing reflector of the preferred embodiment of the presentinvention. Without such reflective silvering an appropriatelydimensioned surface would provide the refractive image-divider earliernoted.

It should again be pointed out that the dimensions of the schematicdrawings have been sealed in greatly exaggerated proportion for thepurpose of clarity. For example, withrespect to FIG. 8, the actualcurvature of reference surface 82 over a practical plurality of facetsets, e.g., 12 sets as earlier described, would be such as to form a.nearly flat image divider.

As can be seen from FIG. 6, a preferred arrangement embodying thepresent invention may result in a slight inclination of scale 62 withrespect to the optical axis of imaging lens 76. However, by means of acompensating inclination in the disposition of image divider 75, thislatter element may be readily located in the plane of best focus of thescale image. As an additional advantage in such an arrangement, theinclination of divider allows an offset of photocell bank 77 and ensuresan uninterrupted light path in module 70. Any variation in lateraldimension of the scale image as a resultof the noted inclination of theimage with respect to the scale may readily be compensated by theintroduction of a similar dimension variation in the facets of imagedivider 75, e.g., as a tapering of the individual facets.

In operation, the present invention provides a series of phase-displacedelectrical signals which may be used in known electronic counter anddisplay devices, such as earlier noted, to yield directly, throughappropriate calibration, indications of linear displacement in toolingor measuring apparatus. If desired, scales of angular dimension may beemployed with the invention to provide direct read-out theodolites orangle measuring systerns.

The foregoing description has been, for the most part, directed toembodiments of the invention which provide the highest degree of signalefficiency, such as where every optical element of a group constitutinga periodic set in the image divider is capable of directing light to therespective photocells in the system, thus utilizing all available lightin the scale image. It will be readily apparent, however, that somearrangements embodying the present invention might be constructed insuch a manner as to include a substantially inactive optical element,for example an opaque facet, in each image divider set in order merelyto obtain the basic assymetry of image period division necessary toestablish signal phase relationships useful in directional counters. Asystem of this type could thus provide an effective device employing apair of photocells where loss of light efficiency is a practicalcompromise to economy. In any event, such embodiments are nonetheless tobe considered within the scope of the invention set forth in thefollowing claims. What is claimed is:

1. in an electro-optic displacement measuring device comprising meansfor deriving a pattern of regularly varying light intensities from ascale element having a regular array of optically discernible indicia,said pattern being positionally displaced with displacement of saidscale element; means for distributing said light pattern among aplurality of photocells variously positioned with respect to said lightpattern so as to generate a plurality of electrical signals varying inpredetermined phase relationship with said pattern displacement; andelectronic counter means utilizing said plurality of signals to indicatethe extent of said scale displacement; the improvement in saidpattern'deriving and light-distributing means which comprises:

a. means for imaging a substantial number of indicia in said scale arrayat an image plane; and b. image-dividing and light-directing meanscomprising a multiplicity of contiguous reflective elements situatedsubstantially at said image plane and arranged seriatim in the directionof periodicity of said scale image, said multiplicity of elements beingfurther arranged in a sequence of groups wherein each group comprises aregular plurality of said elements and is of a dimension substantiallyequal to a' period of said scale image, and respective elements in eachgroup being arranged to direct incident light comprising said scaleimage to different respective ones of said plurality of photocells.

1. In an electro-optic displacement measuring device comprising meansfor deriving a pattern of regularly varying light intensities from ascale element having a regular array of optically discernible indicia,said pattern being positionally displaced with displacement of saidscale element; means for distributing said light pattern among aplurality of photocells variously positioned with respect to said lightpattern so as to generate a plurality of electrical signals varying inpredetermined phase relationship with said pattern displacement; andelectronic counter means utilizing said plurality of signals to indicatethe extent of said scale displacement; the improvement in saidpattern-deriving and light-distributing means which comprises: a. meansfor imaging a substantial number of indicia in said scale array at animage plane; and b. image-dividing and light-directing means comprisinga multiplicity of contiguous reflective elements situated substantiallyat said image plane and arranged seriatim in the direction ofperiodicity of said scale image, said multiplicity of elements beingfurther arranged in a sequence of groups wherein each group comprises aregular plurality of said elements and is of a dimension substantiallyequal to a period of said scale image, and respective elements in eachgroup being arranged to direct incident light comprising said scaleimage to different respective ones of said plurality of photocells.